all: update to Go 1.20, use strings.CutPrefix/Suffix instead of our fork

Updates #7123
Updates #5309

Change-Id: I90bcd87a2fb85a91834a0dd4be6e03db08438672
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

disco/disco_test.go

@@ -7,11 +7,11 @@ import (
 	"fmt"
 	"net/netip"
 	"reflect"
+	"strings"
 	"testing"
 
 	"go4.org/mem"
 	"tailscale.com/types/key"
-	"tailscale.com/util/strs"
 )
 
 func TestMarshalAndParse(t *testing.T) {
@@ -71,7 +71,7 @@ func TestMarshalAndParse(t *testing.T) {
 		t.Run(tt.name, func(t *testing.T) {
 			foo := []byte("foo")
 			got := string(tt.m.AppendMarshal(foo))
-			got, ok := strs.CutPrefix(got, "foo")
+			got, ok := strings.CutPrefix(got, "foo")
 			if !ok {
 				t.Fatalf("didn't start with foo: got %q", got)
 			}